Enzyme unit

The enzyme unit (symbol U or sometimes EU) is a unit for the amount of a particular enzyme.[1]

One U is defined as the amount of the enzyme that produces a certain amount of enzymatic activity, that is, the amount that catalyzes the conversion of 1 micro-mole of substrate per minute. The conditions also have to be specified: one usually takes a temperature of 25°C[2] and the pH value and substrate concentration that yield the maximal substrate conversion rate.

The enzyme unit should not be confused with the international unit (IU); although it is true that both measure chemical activity and that for many enzymes 1 U = 1 IU[4]:30 (because for many enzymes the existing U was adopted as the newer IU), international units can be defined for the biologic activity of many kinds of substance besides enzymes (for example, vitamins and hormones).